In Level 10: Reclaimed, when the objective is "Destroy the AA guns (3 of 3)", you will reach a point where a solider is using a flamethrower to burn an alien, and "Checkpoint Reached" will appear on the left side of the screen. In this area, simply go down the hill and kill all enemies, then reload the checkpoint, and repeat this as many times as desired. You can get 6,000 XP an hour and complete many challenges for additional XP using this trick. You can do this at any checkpoint in the game, but this is a really good area to kill a lot of enemies and get XP.

In Level 10: Reclaimed, once the third AA gun is destroyed, you will continue through the rocky exterior to a destroyed building. The floor is destroyed just past some ammo to the right of the entrance. Stop before leaving this building. There is a sheer drop after stepping through the exit door, leading into a large open area with unlimited spawning aliens. From the exit looking out into the courtyard, look to the far right. Equip any scoped weapon, and look for the aliens appearing above the spotlight. They will run across the roof before jumping down. If you stay inside the building, and do not drop down into the courtyard, the aliens will never stop spawning. Kill as many aliens as desired to get as much XP as desired and complete challenges. Note: When you run out of ammo, simply reload the previous checkpoint. You will retain any points gained toward challenges and XP.

Legendary weapon locations

Search the indicated locations to find all six hidden legendary weapons belonging to characters from James Cameron's Aliens. Note: Legendary weapons cannot be upgraded, and many of the upgrades for standard weapons can make them better than the legendary weapons.

Hicks' Shotgun

In Level 1, while exploring the gravity well with O'Neal, you will eventually reach the top floor where several aliens will attack. Kill them, and explore the nearby rooms before meeting up with O'Neal. Once you meet up with O'Neal, you will need to defend him from alien attacks while the door is opened. Check out the side rooms on this floor to find armor and the legendary weapon. Its bonuses are faster rate-of-fire and increased firepower.

Gorman's Pistol

In Level 4, while exploring the tunnels with O'Neal, you will eventually reach a door you must cut open to continue. O'Neal will wait beside the door. Instead of cutting it open, check out the path left of him, and look for a table at the base of the steps with the legendary weapon on it. Its bonus is increased firepower.

Hudson's Pulse Rifle

In Level 5, once you have found the elevator, you will ride it up to meet with O'Neal and recollect your weapons. Go straight after O'Neal opens up the door, and check the offices to find the legendary weapon on the floor. Its bonuses are increased firepower, increased accuracy, and doubled clip-size.

Vasquez's Smartgun

In Level 6, after retrieving the enemy personell at the end of the level, you will have to fight some soldiers in an outside area. Once you enter the next building, go down the stairs, and open the door. Look in the left corner of the next room to find the legendary weapon. Its bonus is increased firepower.

Vasquez's Pistol

In Level 9, just as you enter the interior of Hadley's Hope, turn left, and explore the path to find a room with extra armor and the legendary weapon. Its bonuses are increased firepower and increased rate-of-fire.

Frost's Flamethrower

In Level 10, during the objective to destroy the third AA gun, you can find a container with a rocket launcher. Look around inside the same container to find the legendary weapon. Its bonus is increased firepower.

In Level 8: Rampart, when you encounter the pilot of the alien ship, as featured in the Alien movie. Shoot bullets into its skull until a holographic space battle begins playing over its head.

Giant donut Easter Egg

In Level 7: One Bullet, when you get the "Find a way into the Weyland-Yutani Facility to save Bella" objective, follow the stairs to a door, and exit the interior to enter a rocky exterior area with lava flowing down a cliff to your right. Run partway up the path ahead, and look to the right across the chasm with a scoped weapon to find a box of donuts. Shoot them, and they will disappear. Then, look to where your teammates are waiting to see a giant donut appear uphill. It will roll down, launch off the cliff, and explode in the lava.

Spaceballs Easter Egg

In Level 7: One Bullet, go through the facility to the operating rooms with Xeno specimens in glass containers. Continue past a room where test subjects are attached to Facehuggers and through the hallway ahead where you encounter more soldiers. In this area, look for a door with a green light. Explore the hallway to locate an office with the mission's second audio log. Before entering the second room where soldiers attempt to activate alarms, leave, and go around to another hallway leading to the same alarm room. Cruz will wait to rush inside the alarm room. Instead of following him, leave the operating room through the alternate door, and enter the alarm room through the left side of the path. By entering the alarm room this alternate way, it will activate a special Easter Egg. Inside one of the glass containers, look closely to see a tiny top hat and cane where a chestburster should be.
This is a reference to the famous scene near the end of Spaceballs, when the character Kane (played by John Hurt) makes a cameo, in which a chestburster explodes from his chest and dances out of the scene singing "Hello! Ma Baby" — which is a reference to the Warner Bros. character Michigan J. Frog.

In Level 2: Battle For Sulaco, after opening the door with the loader, keep going until you reach the dark area with the Facehuggers. Kill the Facehuggers, then continue and look for a blue glowing switch. Activate the switch, then go back into the room where you came from to find a new door open. There are four painted alien Easter Eggs inside the room. Enter the room to get the "Easter Egg" achievement.

Easy "Fire Drill" achievement

In Level 3: Sulaco Falls, at the very end of the level, you will reach a hangar. The relevant section for this achievement begins as soon as you enter this area. You have two minutes and thirty seconds to activate three switches and enter the dropship. It is recommended to play on the Recruit difficulty. This will allow you to rush into the enemy groups without dying. Use a shotgun to kill them while running to the next switch. After activating all three switches, drop down, and go to the dropship. The mission will end, and you will get the "Fire Drill" achievement during the next cutscene. Note: Make a backup of your saved game so you can try again without replaying the entire level again if you do not complete the area within the required time limit.

Easy "Heavy Lifting" achievement

Start Level 5: The Raven on the Soldier difficulty. Raven is an alien Boss at the end of the level. You will have to defeat him using the powerloader. Press LT for quick punches. Try forcing him into a corner, and keep punching him quickly. Sometimes he will attack you or run away. Since the time is relatively short to get the achievement, it may take a few attempts. Be aware the mission will end as soon as the Boss has been defeated. Thus, pause the game when you automatically jump out of the powerloader, and if you do not get the achievement, reload the checkpoint. You may also want to make a backup of your saved game so you can try it again without replaying the entire level again.

Easy "Mostly Come At Night..." achievement

In Level 5: The Raven, at the beginning you will have to find a way out of the sewers. When you reach the section where sound-sensitve aliens are walking around, look for a switch that powers up a generator. The aliens will run to the generator and explode. Newt's doll is not far from this area. Look for three barrels near a yellow light. The doll head is sitting on one of the barrels.

Easy "Short, Controlled Bursts" achievement

In Level 1: Distress, shortly after fighting the first group of aliens, you will reach the hangar bay. Your comrade will throw a grenade and destroy a dropship. Then, you will have to defend him until he opens a door. Two AI controlled allies are standing behind a barricade. None of the enemies can get behind this barricade to get the achievement. It is recommended to play on the Recruit difficulty. Stay in front of the barricade the entire time. Enemies are much more likely to attack real players than AI controlled ones. You may take a few hits, but the shotgun will kill all aliens quickly. Once the door is open, immediately fall back.

Achievements

Accomplish the indicated achievement to get the corresponding number of Gamerscore points:

Another Day in the Corps! (20 points): Complete Distress.

Bad Feeling About This Drop (20 points): Complete Sulaco Falls.

Quoth the Raven (20 points): Complete The Raven.

Still Got a Job to Do (20 points): Complete Rampart.

Dragged Queen (20 points): Complete Home.

Game Over, Man! (30 points): Complete all Campaign levels on any difficulty.

Not Bad For a Human (40 points): Complete all Campaign levels on Hardened difficulty.

State of the Badass Art (50 points): Complete all Campaign levels on Ultimate Badass difficulty.